An ad hoc network has conventionally been known, which is a radio network autonomously configured by multiple radio terminals each having a relay function. Meanwhile, a technique by which a radio terminal forming an ad hoc network communicates with a radio base station has been proposed (see Patent Document 1, for example). Specifically, the radio terminal communicates with the radio base station via a different radio terminal.
To be more specific, a tree structure network is formed of multiple radio terminals, and has a radio terminal directly communicating with a radio base station, as a root. A lower-level radio terminal is configured to communicate with the radio base station via an upper-level radio terminal, and hence the lower-level radio terminal can communicate with the radio base station even when being outside a radio wave reachable range of the radio base station. Patent Document 1: Japanese Patent Application Publication No. 2003-124876 (pp. 6 to 7, FIG. 7).